browse: More README rendering enhancements

swh/web/browse/views/directory.py

swh/web/browse/views/revision.py

swh/web/browse/views/utils/snapshot_context.py

swh/web/tests/browse/views/test_directory.py

swh/web/tests/browse/views/test_origin.py

swh/web/tests/browse/views/test_revision.py